Measured Free 25-hydroxyvitamin D in Healthy Children and Relationship to Total 25-hydroxyvitamin D, Calculated Free 25-hydroxyvitamin D and Vitamin D Binding Protein

Background: vitamin D deficiency in children is still a global health problem. Measuring free 25-hydroxyvitamin D concentrations could provide a better estimate of the vitamin D status than total 25-hydroxyvitamin D (25(OH)D) levels.

Objective: To assess the relationship between measured free vitamin D (m-f25(OH)D) and calculated free 25(OH)D (c-f25(OH)D), total 25(OH)D, intact parathyroid hormone (iPTH) and other markers of phosphocalcic metabolism. To establish serum m-f25(OH)D concentrations corresponding to a total 25(OH)D > 50 nmol/L which is accepted as vitamin D-sufficiency status in children.

Design: Prospective cohort study.

Setting: January and February 2017 in a Mediterranean population.

Patients: healthy children.

Measurements: m-f25(OH)D and vitamin D binding protein (VDBP) by ELISA. Free 25(OH)D was calculated using the formula described by Bikle.

Results: m-f25(OH)D directly correlated with total 25(OH)D (r:0.804,p < .001), serum calcium (r:0.26,p:0.035), and c-f25(OH)D (r:0.553,p:0.016); and inversely with iPTH (r:-0.374, p:0.002), alkaline phosphatase (r:-0.28, p:0.026), and age (r:-0.289, p:0.018). Total 25(OH)D correlated with the same parameters as m-f25(OH)D except for serum calcium. However, c-f25(OH)D correlated only with total 25(OH)D and VDBP, both included in the calculation formula. Multiple regression analysis showed that m-f25(OH)D variations were independently explained by calcium (β:0.156, p:0.026) and total 25(OH)D (β:0.043, p < .001). The optimal m-f25(OH)D cut-off for discriminating between insufficient and sufficient total 25(OH)D was >9.8 pmol/L (Area Under Curve (AUC): 0.897 (95% confidence interval (CI): (0.798-0.958); p < .001; sensitivity:72.7% (95%CI: 49.8-89.3); specificity: 95.5% (95%CI: 84.5-99.4)).

Conclusions: Directly measured free vitamin D correlated better with markers of phosphocalcic metabolism than total 25(OH)D and c-f25(OH)D in a population of healthy children.